1. Field of the Invention
This invention relates to a wireless medium access control (MAC) technique. More specifically, the invention relates to a distributed wireless access method and apparatus based on a network allocation vector table which is applicable to point to point communication in a distributed wireless network, in order to improve the performances, such as throughput, of the system by solving the hidden/exposed terminal problems occurring in the wireless distributed network.
2. Description of the Prior Art
The wireless medium access control (MAC) technique can be classified into centralized control mode and distributed control mode in terms of control scheme. The centralized control mode means that a certain node which is present in a set of network nodes or a node which is specified according to a certain protocol is responsible for the radio resource allocation in the scope thereof to attain an effective share for the resources. Whereas no node in charge of the centralized control is present in the distributed control mode, and all the nodes in the distributed control mode are only responsible for controlling the information of themselves and commonly enjoying the right for accessing a channel. In such a distributed context, the share of the resources by the nodes is dependent on the design and practice of the distributed medium control method.
The MAC protocol characterized mainly by the centralized control is widely used in the existing WAN (wide area network). For example, a base station is used as a control node in the system such as GSM system or WCDMA system, to be responsible for the efficient access of the nodes within its range. Moreover, an access mode for performing a centralized control using an access point (AP) is also present in the WLAN (wireless local area network). As the coverage and the service demand further increases, the distributed MAC without a centralized control device corresponding thereto is becoming an efficient access mode due to its flexibility, such as an ad hoc network. An efficient MAC protocol is needed by a WLAN and the like to achieve an efficient sharing arrangement for the resources among respective nodes. However, since centralized control device is absent for performing resource allocation in a unified way, respective nodes independently control the transmission of their packets, and the problems due to hidden/exposed terminals occurring in the distributed network have become the main factors for affecting performance. That is, when two transmitting nodes are not within the sensing range of each other, and send packets to a same receiving node independently, a collision will occur between packets from the two different transmitting nodes at the receiving node, thereby leading to the degradation of the system performance. This is referred to as “hidden terminal problem”. When two transmitting nodes are within the sensing range of each other, and their respective receiving nodes are not within the sensing ranges of each other, one of the two transmitting nodes could sense the packet transmission from the other transmitting node and thus inhibit its own packet transmission to the receiving node. Since the concurrent transmission between such two pairs of nodes would not lead to a mutual disturbance, the transmission that should have been performed is inhibited due to the sensing of the packet transmission from the ambient nodes. This leads to a waste of system resources and causes the exposed terminal problem.
Specifically, as shown in FIG. 1A, node B is located within the communication ranges of node A and node C, and node C is not located within the communication range of node A. If node A and node C send their respective dada packet to node B concurrently, a collision of these two data packets would occur at node B. This leads to the hidden terminal problem. As shown in FIG. 1B, node B is located within the communication range of node A, and node C is located within the communication range of node B and node D. Node C can not send a data packet due to the sensing of the data transmission from the ambient node B to node A, although the sending by node C at this time would not cause a disturbance on the data transmission between nodes A and node B. This leads to the exposed terminal problem.
In order to solve the hidden terminal problem to reduce the packet collision in the distributed network, IEEE 802.11 specifies the characteristics of medium access control (MAC) layer and physical layer for WLAN, wherein the MAC layer protocol defines point coordination function (PCF) for contention free period (CFP) and distributed coordination function (DCF) for contention period (CP), based on whether an access point is involved in the communication. As shown in FIG. 2, in the communication context without AP, DCF adopts the carrier sense multiple access with collision avoidance (CSMA/CA) protocol. When a packet arrives at a node, a channel is sensed. If the channel is busy, a backoff procedure is entered until the channel become idle and the idle time is equal to the DCF interframe space (DIFS). After the expiration of the backoff time, a short RTS (request to send) packet is transmitted, which includes a transmitting node address (TA), a receiving node address (RA) and a duration necessary for the completion of the transmission for the subsequent packets. The value of this duration is equal to the sum of the duration time necessary for transmitting its subsequent DATA packet, the time for transmitting a CTS (clear to send) packet and an ACK packet, and the time for three short interframe spaces (SIFS). On the contrary, if it is sensed that the channel is idle and the idle time is longer than or equal to the DCF interframe space (DIFS), this node would immediately send an RTS; and if the channel is idle but it is sensed that the channel is busy when the idle time does not reach DIFS, the node would enter the backoff procedure, and the RTS is sent after the expiration of the backoff time. After the receiving node correctly receives the RTS and waits for a short interframe space (SIFS), a short CTS (clear to send) is replied, which includes a receiving node address (RA) duplicated from TA (transmitting node address) of RTS and a duration necessary for the completion of the transmission for the subsequent packet. Herein, the duration is equal to the subtraction of the time for transmitting the CTS packet and the time for a SIFS from the duration in the received RTS. Upon successfully receiving the CTS, the transmitting node waits for a SIFS and sends a DATA (data) packet. Upon successfully receiving this packet, the receiving node sends an acknowledge packet (ACK) for confirmation.
In addition, 802.11 DCF defines a network allocation vector (NAV). As shown in FIG. 3, all non-receiving nodes or non-transmitting nodes which have received the RTS or CTS (the non-receiving node or non-transmitting node will be explained later) compare the values of the durations in these packets with the current values of the NAV, update the NAV with the larger value, and specify that all the nodes can initiate contention to access wireless channel only when their values of the NAV are zero. By using the handshaking procedure and carrier sense for RTS/CTS/DATA/ACK, and the virtual reservation of radio resources by the NAV, it is implemented by 802.11 DCF that when a pair of nodes are communicating, all the ambient nodes whose values of the NAV are not zero cannot access wireless channels, thereby ensuring a collision-free transmission of the packets.
Although 802.11 DCF has solved the hidden terminal problem, allowing the system performances to be improved as compared with the conventional distributed access modes such as CSMA, the problems such as the exposed terminal still restrict further improvement on the utilization rate of system resources. On one hand, some pairs of nodes which would originally cause disturbance on the current communication, that is, the exposed terminals, are inhibited from transmitting packets. This causes waste of radio resources. As shown in FIG. 4, during the communication procedure between node A and node B, node C which is located within the communication range of node A but not within the communication range of node B sets the NAV upon receiving the RTS packet transmitted from node A to node B, whose value is thus equal to the duration time in the RTS packet necessary for the completion of the transmission for the subsequent packets by nodes A and B. During the transmission of DATA packet from node A to node B, if a packet arrives at node C or a packet stored in its memory is waiting for transmission, the node C cannot send the packet because its value of the NAV is not zero.
On the other hand, the setting for the NAV makes some nodes, which can originally receive data, be inhibited from receiving packets. Specifically, as shown in FIG. 5, during the communication procedure between node A and node B, node F which is located within the communication range of node B but not within the communication range of node A sets the NAV upon receiving the CTS packet transmitted from node B to node A, whose value is thus equal to the duration time in CTS necessary for the completion of the transmission for the subsequent packets by nodes A and B. During the reception by node B of DATA packet from node A, if node F receives a RTS transmitted from a node E to node F itself, node F cannot transmit a CTS to respond the reception of RTS because its value of the NAV is not zero. However, in fact, receiving the data from the node E by node F would not cause a disturbance on receiving the data from node A by node B, thereby leading to the waste of resources and further leading to the unnecessary retransmissions when node E receives no response to the RTS. As such, although the conventional medium access control method based on the NAV reduces the collision caused by the hidden terminal to a certain degree at the receiving nodes, the above problems are still not solved and there is still a space for improving the utilization rate of system resources.
Therefore, a need exists for more efficient medium access control method to avoid the collision caused by the hidden terminal, allowing radio resources to be utilized more efficiently.